Apparatus for producing metal powder

ABSTRACT

Apparatus and process for producing a powder from molten metal by forming a flowing stream of metal from a ladle through a regulating tundish and into an atomizing chamber having an atomizing zone and a collecting zone. An atomizing gas is directed through an annular nozzle or series of annularly disposed gas jets against the stream of molten metal to form metal particles, the gas exiting from the nozzle or jets at supersonic velocity. Matter is injected against the particles prior to atomization and/or between the atomizing and collecting zones of the chamber to cause particle agglomeratio to produce particles of irregular shape and to cool the particles. The injected matter may be an inert gas or fine particles of metal powder. The particles settle in the collecting zone where they are subsequently cooled and transported for further processing.

O United States Patent 1191 [11} 3,752,61 1 Reed et a1. Aug. 14, 1973APPARATUS FOR PRODUCING METAL 3,551,532 12/1970 Laird 425/7 x POWDER2,126,411 8/1938 Powell 65/5 2,860,598 11/1958 Loesche 118/418 e tWilliam R West chfi 2,630,623 3/1953 Chesholm Clal 264/12 x William K.Kinur, Northfield Center; Lfkewmd; Primary ExaminerRobert L. Spicer, Jr.gghert E. Kusuer, Brecksville, all of Atmmey Robert p Wright et [73]Assignee: Republic Steel Corporation, [57] ABSTRACT Cleveland OhmApparatus and process for producing a powder from [22] Filed; Aug. 26,1971 molten metal by forming a flowing stream of metal from a ladlethrough a regulating tundish and into an [21 1 Appl' 175407 atomizingchamber having an atomizing zone and a col- Related [1.8, Application Dt lecting zone. An atomizing gas is directed through an [62] Division ofSE1. NO. 834,368, June 18, 1969, Pat. N6. P F

3,655,831 against the stream of molten metal to form metal part1- cles,the gas exiting from the noule or jets at super- 52 us. c1 425/7,425/10, 264/6 Sonic velocity- Matter is injected against the particles51 Int. Cl. B2211 23/08 Prior to atomization and/or between theatomilins and 58 Field of Sell'ch 425/7, 10, 6; collecting zones of thechamber w cause Particle sg 264/12, 6, 5 1 1y 7 glomeratio to produceparticles of irregular shape and to cool the particles. The injectedmatter may be an 5 References Cited inert gas or fine particles of metalpowder. The parti- UNITED STATES PATENTS cles settle in the collectingzone where they are subsequently cooled and transported for furtherprocessing. 2,460,993 2/1949 LeBrasse et al. 425/7 3,281,893 11 1966Ayers 425/7 6 Claims. 9 Drawing Figures Patented Aug. 14, 1973 73,752,611

3 Sheets-Sheet l Patented Aug. 14, 1973 3,752,611

3 Sheets-Sheet 2? APPARATUS FOR PRODUCING METAL POWDER CROSS REFERENCETO RELATED APPLICATION This application is a division of my co-pendingapplication Ser. No. 834,368, filed 18 June 1969 for APPA- RATUS ANDPROCESS FOR PRODUCING METAL POWDER, now Pat. No. 3,655,837 issued 11Apr. 1972 BACKGROUND OF THE INVENTION The present invention relates toapparatus and process for producing metal powder. In particular, theinvention involves the manufacture and preparation of metal powder byinjecting an inert gas or fine particles of metal powder into the flowof molten metal and/or into the flow of metal particles formed by theatomization of molten metal.

It is known in the art to produce atomized powder which may be compactedinto strip, sheet, plate, or finished items such as gears. In general,atomization is a process which involves the disintegration of a streamof molten metal into individual droplets which are subsequentlysolidified. An atomizing jet of fluid, e.g., water, air or inert gas, isdirected toward the molten stream in order to break up the stream intofine particles, which are cooled and solidified.

There are several problems inherent with conventional atomizationpowder-producing processes. If water or air is used as the atomizingfluid, the resulting particles may contain an oxide inclusion and havean oxide coating on their outer surface. If inert gas is used as theatomizing fluid, the particles formed may have a' regular sphericalshape which does not lend itself to good compaction of the powder.Although water will produce the desired irregularly shaped particles,the high oxide content of the product negates this advantage.Furthermore, the hot particles formed by inert gas atomization whichaccumulate on the bottom of the atomization chamber form a sintered massor cake which must then be ground to obtain particles of usable size forsubsequent consolidation into a compacted product. Frequently, thesintered mass or cake is not sufficiently friable to be readily groundwithout destruction of basic particle shapes. It is therefore desirableto provide a process and apparatus which efficiently produces a metalpowder suitable for subsequent compaction.

BRIEF SUMMARY OF THE INVENTION An object of the present invention is toprovide a method and accompanying apparatus for atomizing molten metalinto a powder comprised of particles of irregular shape and lacking anoxide coating. It is another object of the invention to produce a metalpowder which agglomerates but which can be readily ground into particleshaving optimum compacting properties. Still another object is to providea means for dissipating sufficient heat from the atomized metal toprevent fusion of the particles before they can be collected from thebase of the atomizing chamber. A further object is to provide a nozzlefor an atomizer which does not readily become encrusted and hindered inits operation.

To these and other ends, the instant invention contemplates the processof forming a flowing stream of molten metal, e.g., low-carbon,aluminum-killed, drawing-quality steel, directing an atomizing inert gasinto an atomizing zone against the metal to produce particles, injectingmatter against the particles prior to atomization and/or after theyemerge from the atomizing zone to agglomerate them into irregular shapesand to cool the particles, and collecting the particles in a collectingzone.

In accordance with the invention, the molten metal is poured from aladle into a tunclish, which causesa smooth steady stream of moltenmetal to be introduced into an atomizing chamber. There an inertatomizing gas is directed against the molten metal in an atomizing zoneto separate the molten stream into discrete particles. The gas isdirected toward the molten stream by an annular nozzle means, e.g., asingle annular nozzle or a series of jets annularly disposed about theflow of molten metal and constituting a nozzle, which may be of theconverging-diverging type to produce a stream of gas which leaves thenozzle at supersonic velocity. An included gas angle of about 10 to 12is desirable to prevent the nozzle from clogging if aconvergingdiverging type of annular nozzle is employed, but an in cludedgas angle of less than 40 is preferable when separateconverging-diverging jets annularly spaced are used. The gas pressureshould be about to pounds per square inch and the gas should flow at arate of about 1,200 cubic feet per minute for a molten flow of 150pounds per minute. The diameter of the nozzle (the diameter of theannulus defined by either a single annular pipe or the like forming anozzle or'a series of annularly disposed jets forming a nozzle) ispreferably about three to four times the diameter of the molten metalstream. t

Before and/or after the molten metal has been atomized into particles,secondary matter is injected into the atomizing chamber against themetal to cause the particles to agglomerate into irregular shapes and tocool them. This injected matter may be either an inert gas or fineparticles of powder, e.g., as produced from atomization andrecirculated, or both inert gas and powder. If inert gas is employed, itmay be directed toward the particles between the atomizing andcollecting zones of the atomizing chamber in a direction substantiallytransverse to or against the flow of particles. If

fine particles of powder are employed, the particles I may be containedin a hopper and directed toward or with the flow of molten metal oratomized particles by a nozzle or an impeller. The ratio of injectedfine particles to atomized molten metal is preferably about 151 byweight.

Means including a conveyor are provided near the base of the atomizingchamber for collecting the produced powder before it can become sinteredor caked. A rotary cooler may also be employed near the base of theatomizing chamber.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is an elevational view, partlyin section, of representative apparatus in accordance with the presentinvention.

FIG. 2 is an enlarged partial view of one form of atomizing nozzle ofthe invention.

FIG. 3 is an enlarged sectional view of another form of atomizing nozzleused in the invention.

FIG. 4 is a partial sectional view of still another form of atomizingnozzle used in the instant invention.

FIG. 5 is a broken view of another form of apparatus of the invention. 1

FIG. 6 is a broken view of still another form of apparatus of theinvention.

FIG. 7 is a broken view of a further modification of the apparatus.

FIG. 8 is a broken view of yet another embodiment of the apparatus ofthe invention.

FIG. 9 is a perspective view of a means for removing metal powder fromthe atomizing chamber in accordance with the present invention.

DETAILED DESCRIPTION Referring to FIG. 1 of the drawings, there is shownrepresentative atomizing apparatus 10 in accordance with the presentinvention. The apparatus includes a tundish 12 for receiving moltenmetal, e.g., low carbon, aluminum-killed, drawing-quality steel (termedherein AKDQ steel), discharged from a ladle l4 and providing a smoothflowing stream of molten metal. There is provided an atomizing chamber16 having an atomizing zone 16a into which the stream of molten metalfrom the tundish l2 flows for the purpose of atomization of the metalinto powder. A collecting zone 16b within the chamber 16 receives theproduced powder. Disposed within the atomizing chamber 16 is anatomizing nozzle 20 for directing an atomizing gas against the stream ofmolten metal to atomize the metal into powder. A series of secondarynozzles 22 is provided for injecting additional matter into the chamber16 against the flow of molten metal and/or the produced metal powder inorder to agglomerate and cool the metal powder so that it is suitablefor further processing into metal plate, sheet, or strip, for example.

In accordance with the invention, molten metal contained within theladle 14 is preheated to a sufficient temperature to ensure properpouring. The molten metal is then discharged from the bottom of theladle 14 into .the tundish 12, which should be heated to a temperatureof about 2,800 F prior to pouring (in the case of AKDQ steel) in orderto prevent a frozen melt. The tundish regulates the flow of molten metalfrom the ladle 14 into the atomizing chamber 16. The tundish 12 may befabricated from a refractory material, e.g., alumite (93% A1 0 which hasbeen cast in a mold and fired slowly to about l,400 F, e.g., prior touse. Although a tundish 12 having one feeding nozzle is illustrated, itis possible to employ a tundish which will provide multiple streams ofmolten metal.

Associated with the tundish 12 is an inlet pipe 24 for allowing an inertgas, e.g., nitrogen gas, to be introduced into the tundish. This inertgas prevents oxides from forming in the molten metal prior toatomization. In order to produce a suitable metal powder, the atomizingchamber 16 should also be purged with inert gas for about a 30-minuteperiod prior to atomization at a gas flow rate of about 190 cu. ft. perminute for a cham ber volume of about 315 cu. ft. This inert gasatmosphere reduces oxidation of the produced metal particles.

After the molten metal has been formed by the tundish 12 into a smoothflowing steady stream, it is permitted to enter the atomizing zone 16aof the atomizing chamber 16 where the molten metal at a temperature ofabout 2,9503,000 F is transformed into metal particles by means of astream of inert gas, e.g., nitrogen preferably of about 99.995 percentpurity, directed downwardly from an atomizing gas exit zone 20b againstthe molten metal stream by atomizing nozzle 20. If a nozzle 20 of theconverging-diverging type is employed to create a supersonic flow ofgas, the gas exit zone 20b will be in an area located immediately beyond the restriction but within the orifice of the nozzle 20 and is thepoint where the atomizing gas is at its greatest velocity. The atomizinggas will not maintain its supersonic velocity at the point ofintersection with the stream of molten metal. Several designs ofatomizing nozzles 20 have been found suitable for employment in thepresent apparatus. In any case, the nozzle 20 should provide an includedgas angle (designated 20a in FIG. 1) with the orientation of each portof the nozzle at an angle from the vertical equal to one-half theincluded gas angle and directed toward the central axis of the atomizingnozzle 20. The gas from the nozzle 20 atomizes the molten stream ofmetal into particles of fine size, which thereafter cool and which maybe collected at the bottom of the atomizing chamber 16. It is necessaryto use a nozzle 20 with an included gas angle sufficient to preventcoarse, hot particles from striking back at and tending to stick to thenozzle. Such sticking particles would bridge the nozzle and result in apartial or complete reduction of the stream of molten metal. Anatomizing gas pressure of about to lbs. per square inch with a flow rateof about 1,200 to 1,500 cu. ft. per minute for a molten metal flow ofabout 150 pounds per minute or about 8 to 10 cu. ft. of gas per pound ofmolten metal acted upon is preferred. Lower gas pressures produceextremely coarse particles which form a sintered mass at the bottom ofthe atomizing chamber 16 (due to heat retention in the particles), whilehigher gas pressure in combination with a high included gas angle 20aresults in more particles striking back at the nozzle 20 and thuscausing blockage of the nozzle. Furthermore, it is preferable that thediameter of the atomizing nozzle 20 be approximately three to four timesthe diameter of the molten metal stream flowing from the tundish 12 inorder to reduce the pressure within the nozzle and the tendency of themetal particles to strike back at the nozzle.

FIGS. 2 through 4 show various forms of atomizing nozzles 20 which maybe used to direct the inert gas against the stream of molten metal. InFIG. 2, the atomizing nozzle 20 comprises an annular pipe 26 associatedwith a source of inert gas through an inlet pipe 27: The annular pipe 26may alternatively be comprised of quadrants separated by partitions (notshown); in that case, four inlet pipes 27 would be required to supplyinert gas to the nozzles 20. Such an alternative arrangement allowsgreater control over the flow of inert gas and over the deviation of thestream of atomized metal from the vertical. A number of gas jets 28 aredisposed equally about the perimeter of the annular pipe 26. Each gasjet 28 is oriented at a suitable angle from the vertical (up to 20, foran included gas angle of up to 40) and points toward the central axis ofthe annular pipe 26 such that the series of gas jets 28 circumscribesthe stream of molten metal and the gas cone therefrom intersects thestream at a distance from the nozzle 20 within the atomizing zone 16a ofthe atomizing chamber 16. The optimum dimensions for a series of gasjets 28 have been found to be an included gas angle of about 30 and anozzle effective diameter of about 3 /1 in. for a molten stream of metalapproximately ninesixteenths in. in diameter. Ungrindable coarsematerial is at a minimum when the effective diameter of the jets 28 isthat value. If the diameter is either too large or too small the amountof +8 mesh product (large size coarse material which generally cannot beground finer without flattening or destroying particle shape usingconventional grinding equipment) will increase irrespective of anincrease in the injection rate of secondary matter. A smaller diameternozzle also presents a difficulty in assuring proper alignment betweenthe tundish nozzle and the center of the series of jets 28. Each gas jet28 may be formed from copper tubing (28a) or the like and may beflexible so that the included gas angle and effective diameter of thenozzle might be easily adjusted by bending the tubing. The gas jets 28may be provided with a ball-and-socket connection so that the angle ofthe jets may be easily varied. It is preferred that each jet 28 be ofthe converging-diverging type so that the gas may exit from the jets atsupersonic velocity, as described in more detail below in conjunctionwith the discussion of the nozzle shown in FIG. 4.

It has been found that when employing a series of gas jets 28 foratomizing the stream of molten metal, the larger the included gas angle,to some extent the higher the yield of metal powder and the more coarsethe powder produced, the powder consisting mainly of agglomeratedparticles. However, if the included angle is too large, it is possiblethat the nozzle will become encrusted with metal particles and hencehindered in operation.

In FIG. 3 there is shown a conventional atomizing nozzle 20 comprising ahousing 29 defining an annular internal chamber 30 associated with asource of inert gas through an inlet pipe 31. This nozzle 20 has anannular orifice 32 or series of small apetures for directing the inertgas toward the stream of molten metal which passes through an opening 34in the center of the nozzle 20. It is preferred that the internaldiameter of the annular orifice 32 be approximately 3 inches when thediameter of the molten stream of metal is approximately nine-sixteenthsof an inch, as this tolerates to a greater extent misalignment of thetundish 12 and wandering of the stream of molten metal. It has furtherbeen found that an annular orifice 32 comprising an annulus of about0.02 to 0.04 inches is suitable for atomization, unless a high inert gaspressure can be maintained in which case the annular opening may beincreased. Such a nozzle has the disadvantage of clogging within arelatively short time due to striking back of the metal particlesagainst the nozzle, especially when the included gas angle 20a exceedsReferring now to FIG. 4 of thedrawings, there is shown a portion of anannular atomizing nozzle similar to the nozzle shown in FIG. 3 buthaving an annular orifice 32a of the converging-diverging type. Such anozzle includes a restriction 32b within the annular orifice 32a inorder to produce a flow of inert gas which exits the nozzle at the gasexit zone 20b at supersonic velocity. An atomizing nozzle 20 possessingthe characteristic of supersonic flow is desirable in that it permitsmore effective usage of the inert gas needed to atomize the stream ofmolten metal. It is perferred that such a nozzle have an included gasangle 20a of about 10 to 12, when a gas pressure of 100 to 150 lbs. persquare inch is employed, in order to prevent clogging of the nozzle. Theother dimensions of the nozzle 20 are the same as those for the nozzleshown in FIG. 3. However, with a converging-diverging type nozzle, theintersection of the stream of inert gas with the flow of molten metaloccurs at a point more distant from the nozzle than with a conventionalnozzle; thus there is less strike back of the produced metal particlesagainst the nozzle 20 and hence the clogging problem is substantiallyeliminated. It has been found that a convergingdiverging type nozzle mayproduce a metal powder of intermediate size, e.g., l0 +65 mesh, with alesser amount of fine and coarse material.

It has been found that a larger included gas angle (20a) may be utilizedwhen individual gas jets (FIG. 2) are employed rather than an annularnozlzle from a sin gle pipe (FIGS. 3 and 4). This is probably the resultof relieving the vacuum inside the gas cone present with an annularnozzle at higher includled gas angles.

In connection with obtaining supersonic flow from a converging-divergingnozzle, the following relationships are useful:

The factors used in the above equations are defined as follows:

T absolute stagnation temperature (in chamber T absolute temperature atany given location in the nozzle p stagnation pressure (in chamber 30) ppressure at any given location in the nozzle po gas density in chamber30 p gas density at any given location in the nozzle A area at any givenlocation in the nozzle A* area at Mach Number unity, i.e., area atrestriction or throat of the converging-diverging nozzle since M l wmass flow of gas ratio of specific heats (specific heat of gas atconstant pressure divided by specific heat at constant volume) M MachNumber.

By substituting any number equal to or greater th one for the MachNumber M in the above equations, representing gas flow greater than Mach1 (supersonic gas flow), the relationships of pressures, temperatures,

densities, and areas as noted may be determined so that Equation (4) maybe employed to determine the area at the nozzle exit for any givennozzle throat area and desired Mach Number at the nozzle exit.

For a complete discussion of the known principles of supersonic fluidflows, see Ascher H. Shapiro, The Dynamics and Thermodynamics ofCompressible Fluid Flow (Vol. 1, 1953), especially Chapter 4.

Although the figures show a downward flow of molten metal to theatomization zone, a horizontal flow (not shown) is possible. Theatomizing gas would be directed generally horizontally, rather thanvertically. The trajectory of the atomized particles would be horizontaland then vertical. With such an arrangement the apparatus 10 would notbe as high as the described apparatus which directs the inert gasgenerally downwardly.

One important feature of the invention is that before and/or after themolten metal has been atomized into particles within the atomizing zone16a of the atomizing chamber 16, additional matter is injected into theatomizing chamber 16 in order to cause the particles to agglomerate intoparticles of irregular and varied shape and to cool the metal particlesto prevent them from forming a sintered mass. The additional injectedmatter may take the form of, e.g., an inert gas such as nitrogen gaspreferably of 99.995 percent purity or fine particles of powder producedfrom atomization, preferably by recirculating fine particles separatedfrom the main mass of atomized particles. This additional matter isinjected into the atomizing chamber 16 and directed into the flow ofmetal particles through a series of secondary nozzles genericallydesignated 22 in the drawings. It is possible to utilize both secondaryinert gas and recirculated fine powder, as shown in the representativeapparatus 10 of FIG. 1. Employing the secondary injected matter producesan atomized powder which agglomerates and which forms a mass friableenough to be ground into particles of irregular shape but which does notform a tightly sintered mass in the atomizing chamber 16 nor containmassive oxide coatings or inclusions.

Inert gas, e.g., nitrogen gas, injected into the flow of metal particlesat a pressure of about 30 pounds per square inch has been foundeffective to agglomerate the particles so that they are of the optimumshape for compacting into metal plate, sheet or strip. The inert gas maybe injected into the atomizing chamber 16 through a valve 36 and aplurality of secondary nozzles 22a penetrating into the interior of thechamber 16 and directed substantially transverse to the flow of metalparticles, as shown in FIG. 1. Inert gas may also be directedsubstantially upwardly and against the flow of metal particles throughnozzles 22b (as shown in FIG. 8), in which case the gas also redirectsany fine material dissipating from the flow of metal particles oraccumulating in the collecting zone 16b back toward the flowing stream.Most of the agglomeration caused by the inert gas occurs with coarseparticles rather than with fine particles, since the latter cool morerapidly by virtue ofa higher surface area per unit mass and hence arenot subject to agglomeration. Atomized powder injected with additionalinert gas has been found to be of a lower density and to flow at aslower rate than powder produced without the injection of additionalinert gas. Thus the powder so produced is particularly suitable forfurther processing.

Injection of fine particles of powder into the stream of metal particlesresults in more agglomeration of the latter as well as helps to cool theparticles, and hence reduces the amount of tightly sintered materialformed at the bottom of the atomizing chamber 16, increasing the yieldof metal powder. Moreover a produuct compacted from agglomeratedparticles has high strength. This fine powder is advantageously obtainedby collecting the particles which leave the chamber 16 through an inertgas exhaust vent 37 near the atomizing zone 16a of the chamber 16, theparticles being recirculated at ambient temperature.

As shown in FIG. 1, the additional powder, which may be contained in aplurality of hoppers 38, may be injected into the stream of metalparticles by means of a plurality of secondary nozzles 220 coupled tothe hoppers 38 and penetrating into the atomizing chamber 16, orientedso that the powder flow intersects the stream of metal particles at apoint beneath the atomizing zone. Impellers 40 may be employed to directthe injected fine material into the stream of particles in asubstantially transverse direction. The preferred amount of recirculatedfine powder is about 400-600 pounds for a 500 pound charge of moltenmetal, or a ratio of about 1 part of fines to 1 part of molten metal,but a ratio of about 1.3 produces the lowest amount of sintered mass orcake. The embodiment of the apparatus shown in FIG. 1 produces few largeparticles which are ungrindable into usable powder. But the injection offine particles of powder results in a more coarse product than when nosecondary matter is injected. When no secondary powder is injected, onlyan ungrindable sintered mass and loose atomized powder are produced,whereas the addition of the injected matter results in a grindablesintered mass or cake which when ground may be combined with the loosepowder to effect a high yield. The coarse product also contains a higherpercentage of agglomerates than a fine product, thus resulting in a moreusable product.

As shown in FIGS. 5-7, other apparatus may also be employed to introducefine particles of powder into the stream of molten metal and/or metalparticles. In the device illustrated in FIG. 5, fine particles of powdercontained in the hopper 38 are discharged into streams of inert gas,e.g., nitrogen gas, which enter the chamber 16 at a point beneath theatomizing zone 16a thorugh nozzles 22d directed substantially transverseto the flow of metal particles. With this arrangement, the fine powderis fed by gravity into the stream of inert gas so that both forms ofsecondary matter may be injected into the produced particlessimultaneously.

Fine particles of powder contained within the hoppers 38 may also beinjected directly into the stream of molten metal by means of aplurality of nozzles 22c before the metal has been atomized intoparticles in the atomizing zone (as shown in FIG. 6). This methodproduces the highest percentage of coarse particles, especially when therate of flow of fine particles is increased. The fine particles ofpowder cause the metal particles to agglomerate into particles ofirregular and varied shape and to cool the particles to prevent themfrom forming a sintered mass whether the fine particles are introducedprior to atomization (FIG. 6) or subsequent thereto (FIGS. 1 and 5).With the embodiment shown in FIG. 6, however, no supplementary nitrogenjets or impellers 40 are needed to direct the fine particles into theflowing stream, the gravity feed from the hoppers 38 being sufficient.Injection of fine particles of powder directly into the stream of moltenmetal insures that the secondary matter is thoroughly mixed with themetal.

It is further contemplated to inject fine particles of powder into theflow of molten metal and metal particles both prior to and subsequent tothe atomization operation. As shown in FIG. 7, a plurality of hoppers 38similar to the hoppers utilized in the abovementioned embodimentsdischarge fine particles of powder toward the atomizing zone 16a of thechamber 16. Upon exiting each hopper 38, the fine particles are dividedinto two streams, one of which is directed by means of nozzle 22f towardthe flow of molten metal before it has been atomized and the other ofwhich is directed by means of nozzle 22g toward the flow of metalparticles after atomization. Both streams of secondary particles areinjected in the direction of flow of the molten metal and metal particlestream, so that the injection may be accomplished merely by a gravityfeed, without the utilization of secondary nitrogen jets or impellers40. This method results in a less coarse powder than the methodemploying the apparatus of FIG. 6. However, the particle size increasesas more fine particles of powder are injected before atomization asopposed to post atomization, i.e., as the method more closelyapproximates the device of FIG. 6. Greater control over particle sizeand extent of agglomeration is offered by the FIG. 7 apparatus.

It is apparent that secondary matter may be injected at a plurality oflocations, both before and after atomization, as well as in the zone ofatomization. ITt should also be noted that the zone of atomization mightinclude a series of discrete gas sources spaced one from another in thegeneral direction of movement of the stream of molten level so as toprovide. atomization at a number of levels. For example, in theapparatus of FIG. 7, atomizing gas could be directed not only from thenozzle 20 but also from similar nozzle structures located as the nozzles22f and 22g. Such a plurality of levels of atomization might be usefulfor relatively large streams of molten metal, although the problem ofnozzle clogging might be encountered.

Generally, fine particles of powder less than 65 mesh in size have beenfound most suitable for secondary injection. This fraction of theproduct contains a relatively high percentage of spherical particleswhich, if

subsequently compacted into steel plate, strip, or the like, results inan intermediate product of insufficient strength for further processing.In many test runs it was found that although the use of the 65 meshpowder was effective to produce agglomerates, the amount of fines neededexceeded the amount generated. To maintain a material balance in thesystem, it became necessary to recycle as well a portion of the 8+ 65mesh product. Since this more coarse material does not require furtheragglomeration, it was injected solely to minimize the tendency of theatomized particles to form a sintered mass or cake during the collectingand cooling stages. Use of the more coarse product was particularlyeffective with the embodiment shown in FIG. 7.

Subsequent to the injection of secondary material into the stream ofmetal particles, the particles pass into the collecting zone 16b of thechamber 16. It has been found that the particles which settle in therelative center of the collecting zone 16b form a partially sinteredproduct, which when ground produces particles having a desired irregularshape. Thus the particles taken from the relative center of thecollecting zone 16); are especially suited for compaction into metalplate, sheet, or strip.

In FIG. 1, the collecting zone 16b of the atomizing chamber 16 is shownassociated with a rotary cooler 42. The rotary cooler 42 may be utilizedto transport the powder product to a station for further processing. Asan alternative means of transporting the metal powder from thecollecting zone 16b to the next processing station, an enclosedcontainer 44 may be provided, as shown in FIG. 9. A container 44defining a conveying device is presently preferable as it collects thepro duced powder before the particles become sintered or caked, whichcan readily occur as the powder enters the collecting zone 16b at atemperature of about 970 F. Such a device 44 includes hopper plates 46located at the base of the collecting zone 16b and adapted to dischargethe produced metal powder onto a moving conveyor belt 48 enclosed by ahousing 50. The hopper plates 46 are inclined inwardly so that metalpowder will more readily flow onto the conveyor belt 48. A plu rality ofnozzles 52 may be provided for the injection of further inert gas, e.g.,nitrogen gas, or fine particles of powder onto the hopper plates 46 toprevent the metal particles from adhering to the hopper plates andhindering the flow of material onto the conveyor belt 48. A rotarycooler 42 or a conveyor belt 48 is an integral part of the invention asit provides in conjunction with the secondary injected matter a meansfor dissipating heat from the produced particles so that the particlesdo not form a sintered mass: or cake.

EXAMPLE I In one operation of the process using auxiliary fine particlesof metal powder and a device as shown in the upper portion of FIG. 1, anAKDQ steel was usedto produce a metal powder, the steel having thefollowing characteristics (given as percentages of the melt by weight):

Carbon 0.06 0.08 1 Aluminum 003 0.08 (0.05 desired) Manganese 0.28 0.32Oxygen 0.012 0.030 Nitrogen 0.008 Max. Sulfur 0.02 Max. Phosphorus 0.02Mlax Silicon 0.0] Max.

The atomizing chamber 16 (having a volume of approximately 315 cubicfeet and formed from 12 feet'of culvert sections 5 feet in diameterwelded to a rectangular sheet container 5 feet X 4 feet X 3 feet)waspreliminary purged with nitrogen gas at the rate of cubic feet perminute for about 30 minutes prior tearomization. During atomization thechamber 16 was maintained at a gas temperature of 250 to 300 F and apressure of 1.1 inches of water. 500 pounds of molten pressure of 130pounds per square inch. In addition, secondary fine particles of metalpowder 65 mesh) were injected into the stream of particles below theatomization zone by impellers such as impellers 40 in the amount ofabout 1.3 pounds of fine particles per pound of molten metal and at arate of about 124 lbs. per min.

The product collected from the process was classified into threedistinct categories: atomized fines, atomized powder, and atomizedsinter. Atomized fines exited the atomizing chamber 16 through the inertgas exhaust vent 37. These fines made up 6.7 percent (by weight) of theproduct and contained 0.035 percent oxygen and 0.084 percent carbon. Thecumulative sieve analysis of the fines was as follows (in percent byweight):

+28 mesh +35 mesh 0.28 +48 mesh 2.90 +65 mesh 12.97 +100 mesh 32.25 +150mesh 57.42 +325 mesh 91.00

The bulk of the atomized steel (48.8 percent by weight) was atomizeddirectly into powder (excluding fines). 1.9 percent (by weight) of thispowder was +8 mesh in size and 98.1 percent was- 8 mesh, that size beingarbitrarily chosen to indicate ungrindable sinter. The 8 mesh portionhad a density of 3.72 grams per cubic centimeter and the followingchemical composition (in percent by weight):

Carbon 0.070 Aluminum 0.03

Manganese 0.26

Silicon 0.010 Phosphorus 0.003 Sulfur 0.0 l 7 Nitrogen 0.006 Oxygen0.018

Of the 98.1 percent (by weight) of the powder which was less than 8mesh, the cumulative sieve analysis (in percent by weight) was asfollows:

+14 mesh 5.44 +20 mesh 13.25 +28 mesh 23.94 +35 mesh 34.73 +48 mesh43.86 +65 mesh 53.69 +100 mesh 65.13 +150 mesh 79.66 +325 mesh 96.88

EXAMPLE 2 In another operation of the process using the apparatus suchas shown in FIG. 6, particles of AKDQ steel powder of the same chemistryof the steel of Example I were agglomerated and cooled. The atomizingchamber 16 was of the same dimensions and was initially prepared in thesame manner as explained in Example l. The molten steel was introducedinto the atomizing chamber 16 at about the same parameters as disclosedin Example I. A 4-inch inside diameter nozzle 20 comprising a pluralityof gas jets 28 of the convergingdiverging type (FIG. 2) with an includedgas angle of 30 was employed to atomize the stream of molten metal intoparticles. The atomizing inert gas was of the same type and flowed atabout the same rate and pressure as the gas used in the process ofExample 1. Additional fine particles of metal powder (65 mesh) wereinjected directly into the stream of molten metal by nozzles 22c priorto atomization in the amount of about 0.85 lbs. of fine particles perpound of molten metal and at a rate of about 144 pounds per minute.

Again the product collected from the process was classified intoatomized fines, atomized powder and atomized sinter. The cumulativesieve analysis of the fines was approximately the same as for the finesof Example 1.

The cumulative sieve analysis (in percent by weight) of the producedusable powder (including reground sinter) was as follows:

+10 mesh 0.51 +14 mesh 39.57 +20 mesh 61.43 +28 mesh 76.07 +35 mesh84.96 +48 mesh 90.79 +65 mesh 95.02 mesh 98.78 mesh 99.42 +325 mesh99.84

This method produced a high percentage of coarse particles of irregularshape. Ten pound samples of produced powder flowed for about 54 to 67seconds through a 0.5 inch diameter orifice. Again the powder was foundto be suitable for compacting into steel plate, strip, or the like.

Examples 1 and 2 show that particle injection prior to atomization(Example 2) produces a coarser product than injection followingatomization (Example 1).

EXAMPLE 3 Further operations of the process were conducted usingapparatus such as shown in FIG. 7. Again an AKDQ steel having thecharacteristics of the first example was employed and the atomizingchamber 16 was of the same dimensions and was preliminarily prepared inthe same manner. Moreover, 500 pounds of molten steel were againdischarged from the tundish 12 at a temperature of about 3,100 F and ata rate of about156 to lbs. per minute through a ninesixteenths-inchdiameter feeding nozzle in the tundish. A 3-inch inside diameter nozzle20 comprising a plurality of gas jets 28 of the converging-divergingtype (FIG. 2) with an included gas angle of 30 was employed to atomizethe stream of molten steel into particles. The atomizing inert gas hadabout the same parameters as the inert gas used in the operation ofExample 1. Secondary fine particles of metal powder were injected intoboth the stream of molten metal as by nozzles 22f (injected particles 65mesh) and the stream of metal particles as by nozzles 22g (injectedparticles ---65 mesh).

Again the fine particles produced were of approximately the same size asthe particles produced in Example l. The usable powder produced by thisprocess varied in particle size according to the ratio of powderinjected above the atomization zone to powder injected below theatomization zone. Table 1 indicates the cumulative sieve analysis (inpercent by weight) for various ratios, amount of powder injected, andparticle flow rates as follows:

Table 1 Ratio 0.20 0.30 0.65 1.00 1.36 Total Powder 515 560 610 400 465Injected (1b.) Powder Flow Rate 212 208 228 144 170 (lb. per min.)Particle Size (mesh) Cumulative Sieve Analysis (91: by weight) 8+ 100.13 0.10 0.11 0.27 1.39 +14 7.32 23.33 28.38 37.39 46.97 +20 18.4843.31 48.36 59.39 66.11 +28 33.73 59.43 61.83 74.66 77.26 +35 49.7372.13 71.90 85.49 84.52 30 48 66.16 82.12 80.03 91.93 89.60 65 83.0890.12 87.03 96.35 93.73 +100 96.51 97.29 96.09 99.22 9835 +150 98.4198.64 98.61 99.62 99.20 +325 99.31 99.41 99.61 99.99 99.73

Thus it can be seen that as more particles of powder are injected intothe molten metal prior to atomization as opposed to injection subsequentto atomization, the size of the produced particles increases, resultingin desirable intermediate-size powder. Te n pound samples of usablepowder flowed for about 38 to 61 seconds through a 0.5 inch diameterorifice. Only about 23 pounds of +8 mesh material was produced usingapparatus such as shown in FIG. 7 and after regrinding, these particleswere added to the intermediate particles and included in the cumulativesieve analysis of Table 1. Once again, the process resulted in metalpowder suitable for optimum compacting into metal plate or strip. Thedata in Table 1 shows the control over the constituency of producedpowder possible through varying the ratio of injected particles prior toand following atomization.

EXAMPLE 4 Using apparatus such as shown in FIG. 5 for introducing bothsecondary inert gas and fine particles of powder into the streamof'produced metal particles, agglomeration and cooling of the particlesresulted. An AKDQ steel of about the same nature, in about the sameamount, and flowing at about the same rate-as that used in Example 1 wasintroduced into the atomizing chamber (which was prepared in the samemanner indicated in Example 1). The steel was atomized using aconverging-diverging type nozzle 20 (FIG. 2), the atomizing inert gashaving about the aforementioned parameters of the first example.Additional fine particles of metal powder (65 mesh) were injected intothe stream of metal particles after atomization in the amount of about0.4 lbs. of fine particles per pound of molten metal at a rate of about67 lbs. per minute. The injection was accomplished by gravity feedingthe fine particles into a stream of nitrogen gas flowing at a pressureof about 40 lbs. per sq. in. and directed toward the stream of producedparticles by nozzles 22d.

The fine particles of powder produced were similar in size to theparticles produced in the above examples. The cumulative sieve analysis(in percent by weight) of the produced usuable powder (includingreground sinter) was as follows:

+14 mesh 0.0 +20 mesh 3.78 +28 mesh 12.75 +35 mesh 29.36 +48 mesh 51.01+65 mesh 79.21 mesh 97.51 mesh 98.65 +325 mesh 99.39

A desirable powder of intermediate size was thus produced, the metalpowder being appropriate for further processing.

EXAMPLE 5 Auxiliary nitrogen gas was also employed to agglomerate andcool particles of metal powder produced from an AKDQ steel (the steelhaving the characteristics of the first example) using apparatus such asshown in FIG. 8. The atomizing chamber 16 was of the same dimensions andwas initially prepared in the same manner as in the process using thedevice of Example 1. 500 lbs. of molten steel were discharged from thetundish 12 at a temperature of about 3,l05 F and at a rate of about 172lbs. per minute through a nine-sixteenthsinch diameter feeding nozzle inthe tundish. A 3-inch inside diameter nozzle 20 of theconverging-diverging type (FIG. 4) having a 0.040 inch annulus anddefining an included gas angle of 10 was employed to atomize the streamof molten metal into particles. Nitrogen gas of 99.995 percent puritywas directed toward the stream at the rate of 1,100 cu. ft. per minuteat a pressure of l 15 lbs. per sq. in. to atomize the stream.Additionally, nitrogen gas of 99.995 percent purity was injected intothe stream of particles by secondary nozzles such as 22b at the rate of240 cu.. ft. per minute at a pressure of 20 lbs. per sq. in.

The product of the process was again classified into atomized fines,atomized powder, and atomized sinter. The fines comprised 8.6 percent(by weight) of the product and were of the same content and size as thefines of Example 1.

Initially, about 51.7 percent (by weight) of the product completed theprocess as powder. 0.7 percent (by weight) of the powder was +14 mesh insize and 99.3 percent was -14 mesh. The -l4 mesh portion had a densityof 4.27 grams per cubic centimeter and the following chemicalcomposition (in percent by weight):

Of the 99.3 percent (by weight) of the powder which was 14 mesh, thecumulative sieve analysis (in percent by weight) was as follows:

+20 mesh 1.12 +28 mesh 4.84 +35 mesh 17.32 30 48 mesh 35.30 +65 mesh58.78 +100 mesh 78.06 +150 mesh 90.43 +325 mesh 98.71

The majority of the intermediate sized powder was of irregular shape andcoarse in nature. A 25 cubic centimeter sample of produced powder flowedfor 23.3 seconds through a 0.2 inch diameter orifice. The powder wasfound to be suitable for optimum compacting into steel plate although itwas not highly agglomerated.

The atomized sinter constituted 39.7 percent (by weight) of the product.Of the sintered proudct the bulk could be readily ground into usablepowder so that the total amount of powder was 91.4 percent (by weight).Only about 1 percent of the molten metal was eventually found to beunusable.

In order to test the compactability of the atomized powder, samples ofpowder from various runs were formed into briquettes at a pressure ofabout 80,000 psi in a 1 inch diameter die, that pressure being foundnecessary to keep the briquettes of lesser agglomerated powder intact.Upon applying a load to the briquettes, it was found that thosecontaining a higher percentage of agglomerates withstood a highercompression load without rupture, as the agglomerates allow for particleinterlock upon compression. The amount of compression load endured wasalso found to be a function of the length of time of powder flow and theapparent density of the powder.

Thus, the present invention provides apparatus and process for atomizingmolten metal into a metal powder possessing optimum properties forcompaction into metal plate, sheet, or strip, for example. The inventionallows a metal powder to be produced in agglomerated form, which resultsin stronger particle-to-particle bonds in the compacted product, butwithout particles containing oxide coatings and without a sintered massbeing formed at the base of the atomizing chamber which could not bereadily ground into particles of desired size and shape.

We claim:

1. Apparatus for the production of metal powder,

comprising:

a. tundish means adapted to provide a smooth flowing stream of moltenmetal;

b. an atomizing chamber into which said stream of molten metal flows,containing an atomizing zone and a collecting zone;

c. atomizing means for producing particles of metal from said moltenmetal stream;

d. injecting means for directing particles of the same material as themolten metal against the metal in said atomizing chamber to cause theatomizationproduced particles of metal to agglomerate into irregularshapes and to cool them; and

e. collecting means for receiving said metal particles in saidcollecting zone.

2. Apparatus as defined in claim 1, wherein said injecting meansincludes impeller means for directing said particles of material againstthe metal in said atomizing chamber.

3. Apparatus as defined in claim 1, wherein said injecting meansincludes means for entraining said particles of material in a gas whichis directed against the metal in said atomizing chamber.

4. Apparatus as defined in claim 1, wherein said injecting means injectssaid particles of material against said stream of molten metal.

5. Apparatus as defined in claim 1, wherein said injecting means injectssaid particles of material against the atomization-produced particles ofmetal.

6. Apparatus as defined in claim 1, wherein said injecting means injectssaid particles of material concurrently against said stream of moltenmetal and the atmoization-produced particles of metal.

i I II t

1. Apparatus for the production of metal powder, comprising: a. tundishmeans adapted to provide a smooth flowing stream of molten metal; b. anatomizing chamber into which said stream of molten metal flows,containing an atomizing zone and a collecting zone; c. atomizing meansfor producing particles of metal from said molten metal stream; d.injecting means for directing particles of the same material as themolten metal against the metal in said atomizing chamber to cause theatomization-produced particles of metal to agglomerate into irregularshapes and to cool them; and e. collecting means for receiving saidmetal particles in said collecting zone.
 2. Apparatus as defined inclaim 1, wherein said injecting means includes impeller means fordirecting said particles of material against the metal in said atomizingchamber.
 3. Apparatus as defined in claim 1, wherein said injectingmeans includes means for entraining Said particles of material in a gaswhich is directed against the metal in said atomizing chamber. 4.Apparatus as defined in claim 1, wherein said injecting means injectssaid particles of material against said stream of molten metal. 5.Apparatus as defined in claim 1, wherein said injecting means injectssaid particles of material against the atomization-produced particles ofmetal.
 6. Apparatus as defined in claim 1, wherein said injecting meansinjects said particles of material concurrently against said stream ofmolten metal and the atmoization-produced particles of metal.